28 (twenty-eight) is the natural number following 28 and preceding 28.

In mathematics

Twenty-eight is a composite number and the second perfect number as it is the sum of its proper divisors:. As a perfect number, it is related to the Mersenne prime 28 since. The next perfect number is 28-number), the previous being 6. Though perfect, 28 is not the aliquot sum of any other number other than itself; thus, it is not part of a multi-number aliquot sequence. The next perfect number is 496. Twenty-eight is the sum of the totient function for the first nine integers. Since the greatest prime factor of is 157, which is more than 28 twice, 28 is a Størmer number. Twenty-eight is a harmonic divisor number, a happy number, the 7th triangular number, a hexagonal number, a Leyland number of the second kind (2^6-6^2), and a centered nonagonal number. It appears in the Padovan sequence, preceded by the terms 12, 16, 21 (it is the sum of the first two of these). It is also a Keith number, because it recurs in a Fibonacci-like sequence started from its decimal digits: 2, 8, 10, 18, 28... There are 28 convex uniform honeycombs. Twenty-eight is the only positive integer that has a unique Kayles nim-value. Twenty-eight is the only known number that can be expressed as a sum of the first positive integers, a sum of the first primes , and a sum of the first nonprimes , and it is unlikely that any other number has this property. There are twenty-eight oriented diffeomorphism classes of manifolds homeomorphic to the 7-sphere. There are 28 non-equivalent ways of expressing 1000 as the sum of two prime numbers. Twenty-eight is the smallest number that can be expressed as the sum of four nonzero squares in (at least) three ways:, or (see image).

In science

In astronomy

In sports

In other fields

Twenty-eight is: